In recent years, efforts have been made to develop vertical cavity surface emitting lasers (VCSELs) as small-sized, low power-consumption, and inexpensive light sources for optical communication. VCSELs that can be modulated at a high rate of about 1 to 10 Gbps have already been put to practical use. A well-known structure employed for said high-speed modulation VCSELs is an oxide-confined structure formed by utilizing a steam oxidation process. First, with reference to FIGS. 5(a) and 5(b), the structure of a VCSEL in a first conventional example and a manufacturing process thereof will be explained. FIGS. 5(a) and 5(b) are a plan view of a VCSEL with a mesa-post oxide-confined structure and a sectional view taken along line E-E′ in FIG. 5(a) and illustrating the structure of a half cross section extending from the substantial center of a light emitting area to the outer periphery of its mesa. Reported examples of a VCSEL comprising the oxide-confined structure include, for instance, Non-Patent Document 1: KONDO et al., “Lasing Performance of a Highly Strained GaInNa/GaAs Quantum Well Surface emitting Laser”, Collection of Synopses (Autumn, 2003) for the 64th Annual Meeting of the Japan Society of Applied Physics, 30a-YB-3.
A process for manufacturing the aforementioned VCSEL will be explained with reference to the structure of the half cross section illustrated in FIG. 5(b). First, a laser structure portion comprising a semiconductor layered structure is formed on a substrate 113. Here, the semiconductor layered structure comprises active layer 111, p-DBR (Distributed Bragg Reflector) 109, n-DBR 112, and oxidizable layer 110. The oxidizable layer 110 is made up of an AlGaAs layer typically having a high aluminum content x of at least 0.95 contained in total of III group elements thereof. Position of the oxidizable layer 110 to be interposed is either over or under the active layer, or each of the layers is arranged both over and under the active layer.
Then, the layered structure is masked and selectively etched to form mesa 101. As a result of the etching, a part of the semiconductor layered structure comprising oxidizable layer 110 with the high aluminum content is exposed on a side surface of mesa 101. Then, in the steam-oxidation confining step, a partial area of the oxidizable layer 110 is oxidized to convert the area into an electrically insulated steam oxidized film, and thereby nonconductive oxidized layer 117 is formed. Thus formed is a light emitting area 105 with an oxide-confined structure in which the nonconductive oxidized layer 117 is employed as an insulating layer that narrows the range of current injection into active layer 111. Subsequently, the following are carried: a step of forming dielectric protection film 107 and a buried structure made of polyimide 108 to reduce the electric capacitance of an electrode pad, a step of forming a p electrode and an n electrode, and other steps. As a result, a VCSEL device is completed.
Now, the steam-oxidation confining step and functions of the oxidizable layer 110 will be described. The steam-oxidation confining step is such a step in which the substrate with the mesa formed thereon is exposed in an environment supplied with hot steam or a gas, liquid, or solid which allows water to be generated as a result of chemical reaction for predetermined duration. Thus, oxidizable layer 110 with the high aluminum content is selectively oxidized laterally from the side surface toward the center of the mesa 101 and thus converted into a steam oxidized film, and thereby the nonconductive oxidized layer 117 is formed. At the step, since the other layers are formed of an AlGaAs layer with a lower aluminum content than the oxidizable layer 110, that is, with x of 0.95 or less, the progress of the oxidation thereof is limited to such a low level that has by no means any adverse effects on the manufacture of the devices or on the performance of thus manufactured devices. At the steam-oxidation confining step, semiconductor layers composing a DBR section are oxidized, and thus a part of the semiconductor layers are converted into oxidized films. The extension width of the nonconductive oxidized layers formed in the DBR section is typically one-fifth or less of that of the nonconductive oxidized layer formed in the oxidizable layer 110.
As described above, as a result of the oxidation process, in the formation, in oxidized area 102, the nonconductive oxidized layer 117, made up of an electrically insulating steam oxidized film is formed. On the other hand, the conductivity of non-oxidized area 103 is maintained even after the oxidation process. Thus, during VCSEL driving, as the nonconductive oxidized layer 117 functions as a current barrier, no current flows to oxidized area 102 in which the nonconductive oxidized layer 117 is interposed, but current is injected into the active layer 111 only through non-oxidized area 103. Consequently, lasing light is generated by emissive recombination occurring in active layer 111, into which current is injected through the non-oxidized area 103. In the conventional example illustrated in FIG. 5, the planar shape of the light emitting area depends on that of current injection opening area 105. In the conventional example, in any cross sections passing through the substantial center of current injection opening area 105 in the plan view in FIG. 5(a), the path of current injection is defined by the boundary between the oxidized area 102 and non-oxidized area 103.
Furthermore, the oxidizable layer 110 and nonconductive oxidized layer 117 not only determine the planar shape of current injection opening area 105 but also has a refractive index waveguide function for laser light. That is, the nonconductive oxidized layer 117, made up of the steam oxidized film, has amorphous aluminum oxide-like composition, so that its optical refractive index is lower than that of the oxidizable layer 110 included in the non-oxidized area 103. Such a refractive index distribution optically forms a clad/core structure, and has a function equivalent to a focusing lens, so that it acts as an intensive waveguide structure to concentrate light generated during laser driving within the non-oxidized area 103 and its vicinity.
Such an optical waveguide structure provides improvement in the stability of a mode of lasing and increased overlapping between the current injection opening area and the light intensity distribution area, which improves mode gain, resulting in improvement in the lasing efficiency. Achievement of a high mode gain is very important in realizing high-speed modulation of 1 Gbps or higher. The planar distribution shape of the laser emitting area during VCSEL driving depends on the efficiency of the refractive index waveguide function due to the above-described clad/core structure. In the case of a device with a very efficient refractive index waveguide structure, the spread of light from non-oxidized area 103 to oxidized area 102 is inhibited, and thus, the planar shape of the light emitting area can be considered to be almost the same as that of current injection opening area 105. On the other hand, in the case of a device with a less efficient refractive index waveguide structure, the considerable contribution of the spread of light from non-oxidized area 103 to oxidized area 102 makes the light emitting area larger than current injection opening area 105. In the present invention, on basis of a light emitting power intensity (peak intensity) observed substantially in the center of current injection opening area 105, an area with a light emitting power intensity that is at least one-tenth of the peak intensity is defined as a light emitting area.
Now, the functions and advantages of the oxide-confined structure are summarized.
1) Current limiting function: the current-confined structure can be easily formed by the use of an insulating oxidized layer (nonconductive oxidized layer made up of a steam oxidized film);
2) Refractive index control function: an intensive refractive index waveguide structure can be introduced in the vicinity of the active layer without buried re-grown structure.
Now, a second conventional example of a VCSEL with an oxide-confined structure will be described with reference to FIGS. 6(a) to 6(c). FIG. 6(a) is a plan view of a VCSEL with a trench structure formed by using a plurality of oxidized areas. FIGS. 6(b) and 6(c) are sectional views illustrating the structures of half cross sections passing through the center of the current injection opening area. FIG. 6(b) illustrates a half cross section corresponding to a direction in which the cross section contains no trench (F-F′ half cross section), and FIG. 6(c) illustrates a half cross section corresponding to a direction in which the cross section contains a trench (G-G′ half cross section).
The manufacturing process and basic structure of the second conventional example are similar to those of the first conventional example. Thus, only characteristic parts of the second conventional example will be described below. In the second conventional example, first, instead of the mesa-post, a plurality of radially arranged trenches 201 are formed. In the subsequent steam oxidation step, the oxidized area 102 is formed in each of the plurality of trenches 201 so as to start to extend from a side surface of the trench. At the step, by appropriately selecting the location and size of trench 201 and the size of the oxidized area, the non-oxidized area 103 is formed in the vicinity of the center of the radial trench arrangement and around the outer periphery of the radial trenches when fronts of oxidized area 102 starting to extend from each trench come into contact with each other. Then, to prevent possible current leakage to outer peripheral non-oxidized area 103, high-resistance area 104 is formed by means of proton implantation or the like to form final current injection opening area 105. The structure of the second conventional example is equivalent to that of the first conventional example in that the periphery of current injection opening area 105 is covered with the oxidized area 102 and in that in any half cross sections passing through the substantial center of current injection opening area 105, the nonconductive oxidized layer 117 (oxide-confined layer) functions as a current limiting and refractive index controlling component.
Further, as a third conventional example, reported was such an invention relating to two-dimensional dense integration of a VCSEL with a trench-shaped oxide-confined structure, as disclosed in Patent Document 5: JP 10-229248 A. In the third conventional example, in such planar arrangement that equilateral triangles, squares, regular hexagons, or the like are arranged in a plane so as not to form any clearance, circular trenches are two-dimensionally arranged at the position of each vertex thereof to form an oxide-confined structure.
In similar to the case with the second conventional example, in the case of the third conventional example, when oxidation is performed to the degree that the adjacent trench oxidation fronts overlap, the adjacent trench oxidation fronts two-dimensionally contact each other. As a result, current injection opening areas (non-oxidized areas) the periphery of which is surrounded by the oxidized area are formed in a two-dimensional array. After the oxidation confining step is carried out, the area between the adjacent current injection opening areas is converted to the high resistance area by means of proton implantation or the like. In such a case, the proton implantation is performed for electric separation. Thus, the presence of the proton implantation step has no influence on the optical behavior of each of the current injection opening areas (light emitting areas). The above-described process is used to form a two-dimensional array composed of independently drivable VCSELs, in which array the devices are densely arranged.
The layout of the third conventional example allows the adjacent VCSELs to share the oxidized area and is thus advantageous in integration density compared to the case of dense array composed of mesa-post oxide-confined VCSELs.
Technical documents relating to the present invention include Patent Documents 1 to 5 and Non-Patent Documents 1 to 3 listed below.    Patent Document 1: JP 2003-110196 A    Patent Document 2: JP 2005-45243 A    Patent Document 3: JP 2005-311175 A    Patent Document 4: JP. 2005-310917 A    Patent Document 5: JP 10-229248 A    Non-Patent Document 1: KONDO et al., “Lasing Performance of a Highly Strained GaInNa/GaAs Quantum Well Surface emitting Laser”, Collection of Synopses (Autumn, 2003) for the 64th Annual Meeting of the Japan Society of Applied Physics, 30a-YB-3    Non-Patent Document 2: Michael H. MacDougal, et al., “Thermal Impedance of VCSEL's with AlOx—GaAs DBR's”, IEEE Photonics Technology Letters, Vol. 10, No. 1, pp. 15-17, 1998    Non-Patent Document 3: P. O. Leisher, et al., “Proton implanted single mode holey vertical-cavity surface-emitting lasers”, Electronics Letters, Volume 41, Issue 18, Sep. 1, 2005, pp. 1010-1011